The childhood obstructive sleep apnea syndrome (OSAS) is common, occurring in 1-3% of children. The primary treatment is adenotonsillectomy, which is invasive and associated with morbidity. Thus, alternative therapeutic options would be highly desirable. Several studies have suggested that intranasal corticosteroids (NCS) may be effective in the treatment of childhood OSAS. However, these studies have been limited by small size, lack of randomization and blinding, short-term follow-up, involvement of children with only very mild OSAS, and lack of stratifying for the presence of atopy. We therefore propose a randomized controlled trial evaluating the efficacy and safety of NCS vs placebo in children with mild to moderate OSAS. Our overall hypothesis is that NCS will be safe and efficacious in the treatment of mild to moderate childhood OSAS, particularly in children with asthma/atopy, but will require ongoing maintenance therapy. Specifically, we aim to: (1). Determine the efficacy of NCS vs placebo in treating OSAS in children. We hypothesize that children with OSAS randomized to NCS will have an improvement in OSAS compared to children randomized to placebo, as measured by the apnea hypopnea index (AHI), the primary outcome. We also hypothesize that other polysomnographic parameters, symptoms of OSAS, neurocognitive and behavioral symptoms, quality of life and nasal symptoms will decrease with NCS use as compared to placebo, and that naso/oropharyngeal airway size will increase. (2). Determine which factors modify the response to NCS. We hypothesize that asthmatic/atopic children will have a greater response to NCS than non-atopic children. Furthermore, we hypothesize that the response to NCS will be greatest in asthmatic subjects with a Th2-high phenotype in their airways. In addition, we will determine the response to NCS in obese subjects (3) Determine the long-term effect of NCS vs placebo in the treatment of OSAS in children. We hypothesize that NCS will have a limited duration of action, but that ongoing therapy will result in continued remission of OSAS. (4). Determine the side-effects associated with chronic NCS use in children with OSAS. We hypothesize that NCS in the dosage used will not result in a significant number of serious adverse events. This study will evaluate the real world implications of NCS by evaluating neurocognitive and behavioral outcomes in addition to polysomnographic outcomes. This will be the first study to look at the long-term outcomes of NCS for OSAS, including side-effects. In addition, this interdisciplinary study takes advantage of a group of experienced investigators and consultants, including those with expertise in sleep medicine, genetics, allergy/immunology, otolaryngology, bone health, ophthalmology and endocrinology. The study will ultimately lead to better management of OSAS in children. Innovative aspects of the proposal include evaluation of the mechanisms for NCS in treating OSAS, and the individual factors predicting response to NCS, which may ultimately lead to personalized medicine approaches.